Environmental concerns and governmental regulations require reduced emissions of volatile hydrocarbon fuel vapors into the atmosphere. One source of hydrocarbon fuel vapors is the fuel tanks of vehicles using gasoline or other hydrocarbon fuels with high volatility. Fuel vapor can escape to the atmosphere during the filling of the tanks and usually even after the tanks are filled.
The use of an onboard vapor recovery system to remove excess fuel vapor from the fuel tank is one solution to the problem. Typically, a canister with activated charcoal therein receives fuel vapors through a valve assembly mounted in the top of the fuel tank and communicates with the intake manifold of the vehicle engine for exhausting fuel vapor from the canister during operation of the engine. The valve assembly usually has a valve responsive to the level of fuel in the tank that enables the valve to stay open at a sufficiently low fuel level to permit fuel vapors to flow freely from the fuel tank into the canister. As the fuel level rises during filling to approach a desired maximum level of fuel in the tank, a float is raised to close the valve to prevent liquid fuel from flowing through the valve and into the vapor receiving canister. The closed valve also prevents fuel vapor from flowing into the vapor receiving canister. One such system is disclosed in U.S. Pat. No. 5,579,802.
Some of these systems use a high capacity or high flow rate valve to control the flow of vapor from the fuel tank to the vapor storage canister. Current high capacity or high flow rate valve designs tend to be forced into and held in a closed position, when they should be open, by the vapor pressure in the fuel tank. This prevents the vapor from flowing through the valve and into the vapor storage canister. Thus the purpose of the system is defeated because the vapor is not allowed to pass through the valve and into the storage canister and then may be even discharged to the atmosphere. Further, using a single float responsive to the fuel level in the fuel tank to close the valve maintains the valve closed while the fuel level remains at or near the desired maximum level of fuel in the tank. This is undesirable because the addition of fuel to the tank when the valve is closed will increase the pressure within the tank and can increase the discharge of the hydrocarbon fuel vapors into the atmosphere.